1. Field of the Invention
The present invention relates to a system for supplying secondary air into an exhaust system of an internal combustion engine and, more particularly, to a system for controlling the secondary air supply.
2. Description of the Prior Art
The heretofore known secondary air supply control system had a relief valve operative to keep the secondary air in a secondary air supply line at a pressure below a predetermined pressure and also had an air change-over valve operative to direct the air from an air pump selectively to the engine exhaust system or into the atmosphere or back into an air cleaner in accordance with the engine operating conditions. The relief valve was designed to be opened to release the pressurized air at a relatively low pressure level so as to increase the operative life of the air pump.
The percentages of HC and CO contents of engine exhaust gases, in general, are increased when engine speed is abruptly accelerated or decelerated. The solution to the increase in the HC and CO emission at the engine acceleration relied substantially entirely upon the secondary air supply, although the solution to the HC emission increase at the engine deceleration relied upon other control systems. At the engine acceleration, therefore, it was required to supply the secondary air into the engine exhaust system as much as possible. However, because the relief valve in the secondary air supply line was opened to release the secondary air into the atmosphere when the secondary air pressure exceeded the predetermined level, as discussed above, the engine exhaust system was not supplied with a sufficient amount of secondary air during engine acceleration operation, despite the fact that the air pump was operated to discharge sufficient secondary air.
In an attempt to solve the discussed problem, there has been proposed a secondary air supply control system which is operative at an abrupt engine acceleration to supply a secondary air of an amount larger than required during normal engine operation. This secondary air supply control system is disclosed in Japanese Utility Model Laid-Open Publication (Pre-Examination Publication) No. 50-61207 published on June 5, 1975. The system includes a relief valve actuator comprising a diaphragm which defines a chamber into which engine intake vacuum is introduced during normal engine operation to keep the amount of secondary air supply to the engine exhaust system substantially related to the engine intake air. The system is arranged such that, at an abrupt acceleration operation of the engine, the diaphragm chamber in the relief valve actuator is supplied with pressurized air from an air pump to close the relief valve so that all the air from the air pump is fed into the engine exhaust system. However, the supply of all the air from the air pump into the exhaust system would, in some instances, cause the problem of excess secondary air supply which leads to the overheating of exhaust gas purification device such as afterburner or three-way catalyst. According to the system disclosed in the Japanese publication referred to above, moreover, the secondary air supply at the increased rate is continued for all the engine acceleration operation period. Thus, if the acceleration operation lasts more than a predetermined time period, there would also be caused the problem of overheating of the exhaust gas purification device.